Electromagnetic energy seal



May 4, 1965 I R. lRoNFlELD 3,182,164

ELECTROMAGNETIC ENERGY SEAL Filed Feb. 28, 1962 2 Sheets-Sheet lINVENTO? RICHARD [ROI/FIELD HY www. M

A TTORNEY May 4, 1965 R. iRoNr-'IELD ELECTROMGNETIC ENERGY SEAL 2Sheets-Sheet 2 Filed Feb. 28, 1962 A TT ORNE Y United States Patent O3,182,164 ELECTROMAGNETIC ENERGY SEAL Richard Ironfield, Somerset,Mass., assignor to Raytheon Company, Lexington, Mass., a corporation ofDelaware Filed Feb. 28, 1962, Ser. No. 176,379 8 Claims. (Cl.219-1tl.55)

This apparatus relates to an electromagnetic energy seal, and, moreparticularly, to an energy seal for electronic heating apparatus whichseal may be mounted on either the doors or walls of the electronicheating apparatus to permit access therein when the electronic heatingapparatus is not radiating and to provide a seal to prevent leakage ofradiation from the electronic heating apparatus.

Prior energy seals in connection with electronic heating apparatus inthe main comprise spring contacts on the door or front of the electronicheating apparatus oven or cavity. The spring contact method has beenfound to have serious inherent shortcomings in that over a period oftime, particularly if the springs are not kept meticulously clean,contact becomes poor and leakage of R-F radiation occurs. In addition,wherever metal-to-metal contact occurs and there is any appreciablediscontinuity in the mating surfaces, arcing has been known to occur andrapid deterioration of the seal takes place.

The present invention eliminates the aforementioned problem in thefollowing manner. A nonmetallic spacer is provided between the wallsdefining an access opening into the electronic heating apparatus and thecover or door enclosing said access opening. This spacing creates a gapbetween the walls defining the access opening and the cover or doorenclosing said access opening so that there is no metallic contact in aplanar area surrounding the access opening, thus eliminatingobjectionable arcing difliculties inherent in metal-to-metal seals. Thisgap can be looked upon as the interior of a parallel plate transmissionline, one plate being the wall surface adjacent to the door surface, theother plate comprising the door surface. The origin of the parallelplate transmission line structure is the point at which the wall surfaceis no longer parallel to the door surface, that is, where the wallsurface turns inward to the interior of the electronic heatingapparatus. The present invention prevents escape of electromagneticenergy through this gap by providing a cavity in one or more surfaces ofthe parallel plate transmission line, said cavity having a terminatingconducting surface located an integral number of onehalf wavelengthsfrom the origin of the gap. The terminating conducting surface thuspresents a low impedance across the transmission line at approximatelyone-half wavelength from the gap origin. This low impedance, which isessentially a short circuit, is rellected or transformed aquarter-Wavelength back into an extremely high impedance. transformedback another one-quarter wavelength into a low impedance at the originof the transmission line, thereby approximating a short circuit at theorigin which provides an energy seal without need for an actual metalliccontact and thus prevents leakage of electromagnetic energy from theinterior or oven portion of the electrical heating apparatus, whilestill providing a convenient access thereto when the heating apparatusis not radiating.

In turn, the high impedance is reflected or' ice It should be noted thatthe term wavelength used here and as used hereafter refers to thewavelength of the electromagnetic energy originating in the electronicheating apparatus as it travels down the medium in the gap between thewalls of the heating apparatus and the door or cover. For example, thisinvention utilizes a magnetron heating apparatus which radiates at afrequency of substantially 2450 megacycles per second, corresponding toa wavelength of approximately five inches in air. However, since it maybe desirable for compactness, etc., to lill the aforementioned gap andcavity with a dielectric medium which will decrease the effectivewavelength of the radiating energy, the term wavelength herein is notthus limited to the wavelength of the radiation source as propagatedonly in air.

Further objects and advantages of this invention will become moreevident from examination of the drawing, wherein:

FIG. l is a front elevation view of a heating apparatus embodying thepresent invention;

FIG. 2 is a cross-sectional view taken along the line 2 2 of the heatingapparatus of FIG. 1;

FIG. 3 is an enlarged section View taken along the line 3 3 of theheating apparatus of FIG. l;

FIG. 4 shows an alternate embodiment of the present invention as used ona hinged door frame;

FIG. 5 shows an alternative embodiment of the present invention in whichthe cavity is mounted in the electronic heating apparatus wall surface;

FIG. 6 shows an alternative embodiment of the present invention in whichthe door extends into the interior of the electronic heating apparatus;and

FIG. 7 shows an alternative embodiment of the present invention whereinthe gap between the wall and the door is filled with a dielectric mediumhaving a dielectric constant greater than unity, thereby allowing thedimensions of the cavity to be proportionately decreased.

Now referring to FIGS. l and 2, a hollow rectangular oven-like enclosureis made of a suitable metal and has rather thin walls; enclosure 1 isadapted to serve as the oven of the cooking apparatus, Oven 1 is adaptedto have a container 2 of food placed therein, said container resting onthe bottom wall of the oven, while the food therein is being cooked. Inorder to allow access to the interior of the oven for placing the foodtherein and for removing the food" therefrom, a rectangular aperture 3is provided in the front wall of the oven 1 near the lower end of saidwall, this aperture being closable by means of a sliding metal door 4attached to tracking mechanisms 2l) and 22, to be described more indetail hereinafter. When door 4 is closed, the enclosure 1 is entirelyclosed, except for the opening 12 for the exciting means to be describedhereinafter.

Numeral 5 generally designates an electron discharge device of themagnetron type, which includes, for eX- ample, an evacuated envelope `6,made of highly conductive material, such as copper, and provided with aplurality of inwardly directed, radially disposed anode vanes 7. Thearrangement is such that each'pair of adjacent anode vanes 7 forms,together with that portion of the envelope lying therebetween, a cavityresonator whose natural frequency is as is well known to those skilledin the art, a function of the geometry of the physical elements makingup the same. For the purposes of the present invention it is desirablethat the dimensions of each such cavity resonator be such that thewavelength of the electrical oscillations adapted to be generatedtherein has a predetermined value, for example, of the order of fiveinches. Wavelengths of this order lie in the microwave region Vof thefrequency spectrum. However, this invention is equally applicable toradio frequency energy of longer or shorter wavelengths, suchwavelengths lying within or without the microwave region.

`Centrally located in envelope 6 is a highly electronemissive cathodemember 8, for example, of the wellknown alkaline-earth metal oxide type,said cathode member being provided with conventional means (not shown)for raising the temperature thereof to a level sufi'icient forthermionic emission.

The electron discharge device i5 completed by magnetic means (not shown)for establishing a magnetic field in a direction transversely of theelectron path between the cathode and anode members thereof.

Magnetron 5 is energized from any suitable source (not shown) and whenso energized delivers microwave energy or high frequency electromagneticwaves to a coaxial transmission line 9, the inner conductor 10 of whichis coupled to oscillator 5 by a loop 11, and the outer conductor ofwhich is connected to envelope 6. The inner conductor v10 of line 9extends directly into the interior of oven 1 through a suitable opening12 provided in the rear wall thereof, while the outer conductor of saidline is connected to the rear wall of the oven 1 by a suitable fasteningmeans 13. Opening 12 is preferably centered with respect to the verticalside walls `of the oven, as shown in FIG. 1, and is preferably somewhatabove the horizontal mid-plane of the oven, as shown in FIGS. 1 and 2. Astirrerj not shown, as described in United States Patent 2,700,092VissuedA to W. M. Hall, et al., January 18, 1955, may be mounted on theceiling structure"V of oven 1.

The door 4 is comprised of an outer skin of aluminum.

24 wrapped around a frame 26, which consists of four sections ofaluminum channel cut at AIS-degree angles and seam-welded together toform a picture framelike structure .completely encompassing the cookingoven aperture 3. A handle y15 is attached to door 4 to enable manualopening and closing thereof.

As best shown in FIG. 3, a second skin of aluminum 36 extends across theinner surface of the door structure to reinforce the door 4 andcompletely enclose the inside of the frame 26. Still referring to FIG.3, the track mechanism of FIG. 1, items and 22, can be seen to comprisenylon-bearing surfaces 32 and 33 inserted on the'outer flange of thecooking oven walls 341 and 35, around which are wrapped guide rails 36and l37 which are .coupled between the aluminum skin A24 and the frame26 and secured thereto .by sheet metal screws 38 and 39. Guide rails 36and 37 are provided with slotted holes, not shown, at the scre-wsecuring points to allow adjustmentV of the sliding mechanism asrequired, to compensate for tolerance buildup. In accordance with thisinvention, dielectric spacers d@k and 41 are inserted into frame 26,thereby preventing metal-to-metal contact between the metallic door 4and the metallic'oven walls 34 and 35, and accordingly creating gaps 42and 43 between the two surfaces. v

The details of the metal frame 26 can best be illustrated with referenceto the lower portion of FIG. f3, but it Y should be remembered that thisframe extends` completely around the periphery of door o andcorresponding surfaces of frame 26 will be numbered in the upper portionof FIG. 3 accordingly. The metal frame26 has a cavity 44' therein,defined by the interior Wall sunfaces 46, 48, 5t) and 52. In accordancewith this invention, wall surface 46 is adapted .to be disposed adistance of approximately an integral number of one-half wavelengthsfrom the origin 56 of the gap between the door 4 and the oven 1.Furthermore, the opening S4 intoV theV cavity 44 is 1oil cated so thatthe midpoint of the opening 54 is approximately one-quarter wavelengthfrom rwall surface 46. The surface 46 thus presents a conductivetermination to electromagnetic wave energy emanating from the ovenheating apparatus. This effective short circuit is reflected back as anopen circuit to the midpoint of opening 64, a quarter wavelength away,and again another quarter wavelength away to a short circuit, until theorigin 56 of the gap 43 is reached an integral number of one-halfwavelengths from the terminating conducting surface 46, at which point ashort circuit is created between the parallel plate transmission lineformed by exterior oven wall surface S8 and door frame surface 60. Itcan thus be seen that an effective electronic heating apparatus seal hasbeen provided in accordance with this invention, which allows a gapbetween the electronic heating apparatus metallic walls and the metallicdoor or cover thereto, in order to prevent aroing, while at the sametime providing a non-metallic effective short circuit at the gap originto prevent egress of the electromagnetic energy permeating the ovenchamber.

One embodiment of the invention, as shown in FIGS. l, 2 and 3, utilizedthe following dimensions for optimum results on a microwave cooking ovenwitha magnetron heating device operating at substantially 2450megacyles: six nylon buttons, as shown in FIG. 1 were displaced aboutthe circumference of frame 26, having a thickness of .050 inch providinga .050 inch gap between the wall and door surfaces, surface S0 was 1.2inches wide, thus providing'substantially a one-quarter wavelengthdistance between surface 48 and exterior oven wall sur-face 5S. Surface48 was 1.562 inches wide and surface 52, .875Y inch wide, the opening 54being thus .687 inch wide and the midpoint thereby located 1.2185 inchesfrom terminating conducting surface 46 or substantially one-quarterwavelength therefrom. Surface 46 was disposed laterally adjacent to theorigin 56 of gap 43, as shown in FIG. 2, so that the distance from theorigin 56 to surface 46 was substantially one-half wavelength. VTheresultant configuration was found to provide an effective seal under alloperating conditions and was shown to have maintained the radiationleakage below 10 inw/cm.2 (the accepted level of radiation) in allcases.

In accordance with this invention, therefore, it has been found that inorder to achieveV the maximum leakage prevention itis preferable todefine the cavity interior surl faces with more specificity VthanVmerely locating the terminating conducting surface an integral numberof 1/2 wavelength from the origin 56. For example, the width andlocation of the opening 54 contributes in establishing therequiredenergy sealing properties. This dimension has been specified by locatingthe mid-point of the open- Y ing substantially Va Mr wavelengthfrom theterminating conducting surface 46. The width of said opening has beenspecified in proportion to the other cavity dimensions by requiring theinner cavity cross-sectional perimeter comprising the width of surfaces52, 46, 46 and 54) to be equal to or less than one wavelength, inVadditionY shows another alternative embodiment in which the cavity`seal is enclosed in a flush-mounted door frame 65, each ofsaidembodiments operating in substantially the same manner.' FIG. 7shows Ya further embodiment wherein the cavity ,4l-l1 and the gap 43 isfilled with a dielectricA material 66, such as polystyrene, which actsto shorten the physica -length of the one-half wavelength with respect`to a similar structure having air therein. Y Thus, the cavity dimensionsmay be correspondingly decreased in order to make the seal more compactfor ease in fabrication and appearances. Accordingly, it is to beunderstood that the above-described arrangements are illustrative of theapplication of the principles of the invention. Numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention. Accordingly, it isdesired that the invention not be limited to particular details of theembodiment disclosed herein except as defined in the appended claims.

What is claimed is:

1. In combination;

an enclosure;

means for supplying electromagnetic waves to the interior of saidenclosure, said enclosure having an access aperture therein;

a door for closing said aperture;

spacing means preventing metallic contact between said door and saidenclosure in a planar area surrounding said access aperture, uponclosure thereof, said spacing means thereby providing a gap between saiddoor and said enclosure;

and a cavity situated in one of the gap defining surfaces having a top,bottom, and a plurality of side walls, said top wall having an openingat one end which is located substantially 1A wavelength from one of saidside walls defining a terminating conducting surface, said terminatingsurface being located substantially an integral number of 1/2wavelengths from the origin of said gap.

2. In combination:

an enclosure;

means for supplying electromagnetic waves to the interior of saidenclosure, said enclosure having an access aperture therein;

a door for closing said aperture;

spacing means preventing metallic contact between said door and saidenclosure in a planar area surrounding said access aperture, uponclosure thereof, said spacing means thereby providing a gap between saiddoor and said enclosure;

a cavity having a inner cross-sectional preimeter substantially equal toone wavelength, a terminating conducting surface therein which islocated substantially an integral number of 1/2 wavelengths from theorigin of said gap, a surface substantially perpendicular to saidterminating conducting surface and located less than or equal to 1Awavelength from the gap defining surface opposite the gap definingsurface in which said cavity is situated.

3. High frequency apparatus comprising:

a metallic enclosure;

means for supplying high frequency electromagnetic waves to the interiorof said enclosure, said enclosure having an access aperture therein;

a metallic door for closing said aperture, said door,

upon closure thereof, being prevented from contacting the metallicenclosure in a planar area surrounding said access aperture bynon-metallic spacing means, said spacing means thereby providing a gapbetween said door and said metallic enclosure;

and a dielectric filled metallic cavity situated in one of the gapdefining surfaces having a terminating conducting surface therein whichis located substantially an integral number of 1/2 wavelengths from theorigin of said gap.

4. High frequency apparatus comprising:

a metallic enclosure;

means for supplying high frequency electromagnetic waves to the interiorof said enclosure, said enclosure having an access aperture therein;

a metallic door for closing said aperture, said door, upon closurethereof, being prevented from contacting the metallic enclosure in aplanar area surrounding said access aperture by non-metallic spacingmeans, said spacing means thereby providing a gap between said door andsaid metallic enclosure, said gap being filled with a dielectric mediumhaving a dielectric constant greater than air;

and a metallic cavity lled with a dielectric medium having a dielectricconstant of greater than air situated in one of the gap definingsurfaces, said cavity having a terminating conducting surface thereinwhich is located substantially an integral number of 1/2 wavelength fromthe origin of said gap.

5. An electromagnetic energy seal comprising:

a box-like metallic cavity having a slit along one surface and extendingtransverse thereto, said box-like cavity being formed in the manner of apicture frame to define a structure co-extensive with the shape of thesurface to be sealed, said box-like metallic cavity having a firstsurface positioned perpendicular with respect to said slit and located adistance approximately equal to 1A wavelength from the midpoint of saidslit, said box-like cavity having a cross-sectional perimeter excludingsaid slit dimension substantially equal to one wavelength, said box-likecavity having a non-conductive spacer coupled to the exterior of theslotted surface, said non-conducting spacer so dimensioned as to providea gap between the surface to be sealed and the metallic cavity, thestart of said gap being located a distance approximately equal to anintegral number of 1/2 wavelengths from said first recited surface.

6. In a high frequency apparatus:

a sliding door;

an electromagnetic energy seal comprising;

a box-like metallic cavity having a slit along one surface and extendingtransverse thereto, said box-like cavity being formed in the manner of apicture trarne to define a structure co-extensive with the shape of thesurface to be sealed, said box-like metallic cavity having a firstsurface positioned perpendicular with respect to said slit and located adistance approximately equal to 1A wavelength from the midpoint of saidslit, said boxalike cavity having a crosssectional perimeter excludingsaid slit dimensions substantially equal to one wavelength, saidbox-like cavity having a non-conductive spacer coupled to the exteriorof the slotted surface, said nonconducting spacer so dimensioned as toprovide a gap between the surface to be sealed and the metallic cavity,the start of said gap being located a distance approximately equal to anintegral number of 1/z wavelengths from said first recited surface, saidseal mounted on the periphery of said sliding door.

7. In a high frequency apparatus:

a hinged door;

an electromagnetic energy seal comprising;

a box-like metallic cavity having a slit along one surface and extendingtransverse thereto, said box-like cavity being formed in the manner of apicture frame to define a structure co-extensive with the shape of thesurface to be sealed, said box-like metallic cavity having a firstsurface positioned perpendicular with respect to said slit and located adistance approximately equal to 1A wavelength from the midpoint of saidslit, said box-like cavity having a cross-sectional perimeter excludingsaid slit dimension substantially equal to `one wavelength, saidbox-like cavity having a non-conductive spacer coupled to the exteriorof the slotted surface, said non-conducting spacer so dimension as toprovide a gap between the surface to be sealed and the metallic cavity,the start of said gap being located a distance approximately equal ot anintegral number of 1/2 wavelengths from said first recited surface, saidseal mounted on the periphery of said hinged door.

7 S. In a high frequency apparatusv an electromagnetic cavity, the startof said gap being located a distance energy seal comprising:approximately equal to an integral number of 1/z a box-like metalliccavity having a slit along one wavelengths from said rst recitedsurface, said seal surface and extending transverse thereto, saidboxmounted on the periphery of an access opening to an like cavity4being formed in the manner of a picture 5 electron heating apparatus.frame to dene a structure coextensive with the shape of the surface tobe sealed, said box-like metal- Reremes Cifie ,by me Exmne lic cavityhaving a first surface positioned per- UNITED STATES PATENTS pendicularwith respect to said slit and located a 2106771 2/38 Southworth 333 83distance approximately equal to 1A wavelength from 10 2241119 5/41Baumbach 333 83 the midpoint of said slit, said box-like cavity hav2585563 2/52 Lewis et al "219 10 55 ing a lcross-sectional perimeterexcluding said slit 2716694- 8/55 Schroede 219 10`55 dimensionsubstantially equal to one Wavelength, said 2794185 5/57 Sichak 333484box-like cavity having a non-conductive spacercou- 2 956 143 10/60 Schau"2"19 10 55 pled to the exterior of the slotted surface, said non- 153092164 9/61 Wilson 333 '84 conducting spacer so dimensioned as toprovide a gap between the surface to be sealed and the metaliic RECHARDM. WOOD, Primary Examiner.

1. IN COMBINATION AN ENCLOSURE; MEANS FOR SUPPLYING ELECTROMAGNETICWAVES TO THE INTERIOR OF SAID ENCLOSURE, SAID ENCLOSURE HAVING AN ACCESSAPERTURE THEREIN; A DOOR FOR CLOSING SAID APERTURE: SPACING MEANSPREVENTING METALLIC CONTACT BETWEEN SAID DOOR AND SAID ENCLOSURE IN APLANAR AREA SURROUNDING SAID ACCESS APERTURE, UPON CLOSURE THEREOF, SAIDSPACING MEANS THEREBY PROVIDING A GAP BETWEEN SAID DOOR AND SAIDENCLOSURE; AND A CAVITY SITUATED IN ONE OF THE GAP DEFINING SURFACESHAVING A TOP, BOTTOM, AND A PLURALITY OF SIDE WALLS, SAID TOP WALLHAVING AN OPENING AT ONE END WHICH IS LOCATED SUBSTANTIALLY 1/4WAVELENGTH FROM ONE OF SAID SIDE WALLS DEFINING A TERMINATING CONDUCTINGSURFACE, SAID TERMINATING SURFACE BEING LOCATED SUBSTANTIALLY ANINTEGRAL NUMBER OF 1/2 WAVELENGTHS FROM THE ORIGIN OF SAID GAP.